I'll upvote this mostly for the clever use of exports and (ab)use of generators.
my build system does the same as that Flyfile and add source maps to my coffee files, looks like this: http://pastebin.com/BtegzqVX
You could argue my polvo.yml file has more lines in it and it's less customisable compared to a script, but still i believe my .yml config is very organised and i can hook up any script to happen afterwards by appending a line to my beautiful makefile.
Not to say, with the "setup" target you could even install node.js or fly or whatever you need before even being able to execute fly.
EDIT: or at least, for me, this yaml-formatted bash script is functionally indistinguishable from a simple makefile.
Thanks! I agree with you. I understand the opinion of the person below, but I think there is room for improvement in this area so I am trying to make the build system the JavaScript community really deserves.